events: iCalendar (RFC 5545) export + 21 tests

ical.sx serializes events to VEVENT/VCALENDAR text for import by standard
clients: UTC basic-format stamps, DURATION (PT#H#M), full RRULE
(FREQ/INTERVAL/COUNT/UNTIL/BYDAY incl. monthly ordinals 2TU/-1FR/BYMONTHDAY)
plus EXDATE/RDATE. Line-oriented (ev/event->ical-lines / ev/events->ical-lines)
with ev/ical-render joining CRLF for the wire format. 332/332 green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/events/conformance.conf

@@ -19,6 +19,7 @@ PRELOADS=(
   lib/datalog/magic.sx
   lib/events/calendar.sx
   lib/events/timezone.sx
+  lib/events/ical.sx
   lib/events/availability.sx
   lib/persist/event.sx
   lib/persist/backend.sx
@@ -49,6 +50,7 @@ PRELOADS=(
 SUITES=(
   "calendar:lib/events/tests/calendar.sx:(ev-calendar-tests-run!)"
   "timezone:lib/events/tests/timezone.sx:(ev-timezone-tests-run!)"
+  "ical:lib/events/tests/ical.sx:(ev-ical-tests-run!)"
   "availability:lib/events/tests/availability.sx:(ev-availability-tests-run!)"
   "api:lib/events/tests/api.sx:(ev-api-tests-run!)"
   "booking:lib/events/tests/booking.sx:(ev-booking-tests-run!)"